In a plain old telephone service (“POTS”) system, a telephone is designed to receive a “ring” signal, typically an AC wave at 20 hertz (Hz) of approximately 40 V RMS, when it is supposed to ring. It will be appreciated that other ring voltages are often used that fall within a range equal to or greater than 40 V RMS. The ringing signal is/was traditionally sent along the twisted pair telephone wire that connects an individual telephone to the telephony network. A typical telephone still uses the higher ring signal voltage, even if the telephony communications are transmitted along non-traditional pathways, such as, for example, community antennae television (“CATV”) coaxial cable, optical fiber cable, or any other network technology that transmits communications signals. When one of these technologies is used, a telephone is typically connected to a line card that provides an interface between the telephone and the communications network. Such a line card typically contains subscriber line interface circuitry (“SLIC”) that is designed to translate a signal from the network protocol to the traditional telephony protocol. A node within the line card circuitry may be maintained at a constant 90-95 V level, and when an instruction to ring is received, the 90-95 V source is directed to the telephone in accordance with a predetermined waveform shape. Thus, when a line card receives a signal that instructs it to cause a telephone connected thereto to ring, it generates a ringing signal of about approximately 40 V RMS, or equal to or greater than 40 V RMS as discussed above, having a waveform shape similar to the waveform of a traditional telephony signal.
A line card may be part of a subscriber's premise equipment (“SPE”) device inside a consumer's home. To deliver power to the SPE, a typical household AC power supply may be used that provides AC current in the range of between approximately 105 V and 230 V. In order to facilitate compatibility with different styles of power outlets and powering schemes, the power supply conductor, or cord, may be terminated with a two prong plug that works in either a two prong receptacle or a three prong receptacle.
Although the use of a two prong plug facilitates compatibility with a wider range of receptacles, a two prong plug does not provide a safety ground path for the device to which it provides power. Thus, the internal circuitry of the device, while being possibly grounded through other means, a coaxial CATV cable, for example, is not safety grounded from the power supply power source. This may cause difficulty in obtaining listing by an independent, product-safety testing and certification organization, such as UNDERWRITERS LABORATORIES, INC.® (“UL®”). Abnormal testing may be required before certification and/or listing of a device that is not safety grounded is granted. This may cause delay in the introducing of a new product that is not safety grounded. In addition to protracted delay for testing, there is the possibility that a device being tested may not meet the certification's standards, thus delaying further introduction of the device into the stream of commerce until the device can be redesigned and retested. Such delay can be costly at best, and result in the stillbirth of a product at worst in the fast changing realm of modem telephony.
Current UL® standards specify that continuous internal voltages within an unearthed device not exceed a hazardous voltage level. If a device, such as a cable modem telephony line card, is not safety grounded, it is subjected to abnormal testing that impose “double insulation” requirements, for example, before the device becomes UL® “listed.” However, if a device's internal voltages that rise above the predetermined safe level, approximately 50-60 volts, for example, are not continuous, but transient in nature, then the device may not be subject to abnormal testing due to internal voltages exceeding the predetermined safe level. Since line cards typically generate a ringing signal voltage of approximately 40 V RMS, or equal to or greater than 40 V, to cause a telephone to ring, an unearthed SPE having a line card that provides a telephone interface may be subjected to abnormal testing, and the aforementioned associated delay, before becoming listed, if an continuous, approximately 90-95 voltage tap, or node, is provided in the circuitry of the line card.
Thus, there is a need for a method and system that provides a 90-95 V ringing signal that is not continuous, but only high (above the predetermined safe level) during ringing, thereby being deemed transient in nature.